Various types of micromechanical devices are known. Such devices include micromechanical spatial light modulators ("SLMs") having pixels formed of electrically addressable, deflectable mirrors or reflectors. SLMs are transducers capable of modulating incident light in correspondence to an electrical and/or optical input. SLMs are capable of modulating the incident light in phase, intensity, polarization, and/or direction.
The present invention relates to SLMs of the foregoing type which are called digital micromirror devices or deformable mirror devices ("DMDs"). SLM DMDs of the type involved herein may be used in a variety of devices, such as printers, imaging systems, xerographic reproduction systems and digitized video systems. See commonly assigned U.S. Pat. Nos. 5,041,851, 4,728,185, 5,101,236 and 5,079,544.
Commonly assigned U.S. Pat. Nos. 5,061,049 and 5,096,279 (hereinafter "'049" and "'279") disclose the structure of, and methods of producing, preferred micromechanical devices, specifically DMD SLMs. In general, micromechanical devices typically include a deflectable or movable mass supported by a deformable beam. According to '049, a DMD SLM may include an array or matrix of relative thick, generally planar, metal mirrors or reflectors, constituting the "mass." Each mirror comprises a layer of aluminum or an aluminum alloy, such as Al (98.5-98.8%):Si(1%):Ti(0.2-0.5%) which is formed by sputtering and selective etching.
The mirrors reside on a relatively thin layer similarly constituted and also formed by sputtering and selective etching. Each mirror is supported by one or more beams. The beams comprise portions of the relative thin layer which extend beyond the boundary of each mirror and are, in turn, ultimately supported by one or more spacers or posts which may be constituted of a photoresist or a metal. The spacers or posts define or are separated by wells beneath the mirrors and into and out of which the mirrors may move when they are selectively deflected. The spacers or posts and the wells are, in turn, formed by selective deposition and removal or patterning of metal, insulative and photoresist laminae.
An undetected DMD mirror may occupy a normal position which may be "horizontal," that is, above its well and generally parallel to a substrate on and in which the DMD is formed. Each normally positioned mirror reflects light incident thereon to a first destination. The mirror is selectively deflectable out of its normal position by the selective application thereto of a predetermined electrostatic attractive or repulsive force. A deflected mirror may be "non-horizontal" or rotated out of the horizontal. Each deflected mirror modulates light incident thereon by reflecting the light to a second destination which depends on the amount of deflection and, accordingly, the presence and/or strength of the applied electrostatic force.
Movement of a mirror out of its normal position deforms its beam(s), storing potential energy therein. The stored potential energy tends to return the mirror to its normal position once the electrostatic force is removed. The beam(s) supporting a mirror may deform in a cantilever mode, in a torsional mode, or in a combination of both modes, called the flexure mode.
The selective electrostatic deflection of the mirrors of an array or matrix thereof is selectively effected by a congruent array or matrix of electrodes located on or in the substrate and on or at the bottoms of the wells. Selected electrostatic force-producing voltages are selectively applied to the electrodes by MOSFET or functionally similar elements and associated electrical components associated with the electrodes. These circuit elements and components are typically formed on and in the substrate by traditional integrated circuit manufacturing processing techniques. Specifically, the MOSFETS or other elements and their associated components, as well as the mirrors, beams, posts or spacers and electrodes are preferably integrally, monolithically formed by typical CMOS or similar techniques in and on a silicon or other substrate.
Extensive testing and analysis of the above-described type of micromechanical device has indicated that the strength of the beams is not sufficiently great to resist relaxation--a phenomenon also known as "creep" or "deformation"--thereof following sustained usage. Such relaxation of the beams results in improper operation of micromechanical DMD SLMs and other similar micromechanical devices. For example, a relaxed beam may be incapable of maintaining its mirror in, or returning its mirror to, the normal position when there is no attractive electrostatic force applied thereto. In a non-normal position, the mirror may reflect incident light to other than the first or second destinations. Thus, relaxation of a beam leads to unintended modulation of incident light. Additionally, even if relaxation does not result in a mirror not properly returning to its normal position, relaxation of the mirror's beam(s) can result in the mirror not deflecting by the appropriate amount upon the application of the predetermined voltage to the applicable electrode. Again, improper modulation of incident light results.
Beams which are stronger than those consisting of aluminum alloys and which are less subject to relaxation are known. For example, early developed SLMs related to the type described above utilized beam-like members comprised of silicon oxide. See U.S. Pat. Nos. 4,356,730, 4,229,732 and 3,886,310. It has also been generally proposed to fabricate the beam(s) of DMD mirrors from materials stronger than, and less subject to relaxation or creep than, the aluminum alloy described above. The use of such materials carries with it, however, the likelihood that the processing sequences and materials (e.g., etchants) presently used to fabricate DMDs, including their addressing circuitry and mechanical elements, would require substantial or radical modification, possibly adding to the complexity of processing resulting in a concomitant increase in the cost of producing DMDs.
Another proposal involves fabricating beams with multiple laminae of aluminum or aluminum alloy alternating with laminae of a stronger, less ductile material, such as alumina. The outer laminae are aluminum or aluminum alloy, so that the majority of the processing steps involving etching remain the same as those described above to produce the traditional DMD structure. Because the alternating laminae are produced by periodically interrupting sputter deposition of the aluminum or aluminum alloy and sputter depositing the stronger, less ductile material, the process is complicated to that extent, and production costs may be increased.
A desiderata of the present invention is the provision of micromechanical devices, such as DMD SLMs, having beams which are stronger and relaxation-resistant, the beams being fabricated without substantially or radically increasing the complexity or cost of the DMD processing sequence.